Method, an apparatus and a computer program product for focusing

1. A method for focusing, comprising:

receiving a first image stack of a first field of view, the first image stack comprising images captured with different focus from the first field of view;

determining, from the first image stack, a first spatial distribution of focus depths in which different areas in the first field of view are in focus;

determining a first local sample thickness and a first sample tilt in the first field of view based on the first spatial distribution of focus depths;

receiving a third image stack of a third field of view, the third image stack comprising images captured with different focus from the third field of view and wherein the first field of view and the third field of view are adjacent fields of view for a second field of view;

determining, from the third image stack, a third spatial distribution of focus depths in which different areas in the third field of view are in focus;

determining a third local sample thickness and a third sample tilt based on the third spatial distribution of focus depths; and

estimating, based on the first local sample thickness, the first sample tilt, the third local sample thickness and the third sample tilt, a focus setting for capturing a second image stack from the second field of view.

2. The method according to claim 1 , wherein the determining the first local sample thickness and the first sample tilt comprises

eliminating effects caused by apparatus-specific measures from the spatial distribution of focus depths.

3. The method according to claim 2 , wherein the apparatus-specific measures comprise one or more of a pre-defined field curvature and a pre-defined optical axis tilt.

4. The method according to claim 1 , further comprising

estimating a first focus setting based on the first local sample thickness and the first sample tilt;

estimating a third focus setting based on the third local sample thickness and the third sample tilt;

averaging the first focus setting and the third focus setting to obtain the focus setting for capturing the second image stack from the second field of view.

5. The method according to claim 4 , further comprising

assigning a weight for the first focus setting based on the distance between the first field of view and the second field of view.

6. The method according to claim 4 , further comprising

assigning a weight for the first focus setting based on a planarity of the first spatial distribution of focus depths.

7. A method for focusing, comprising:

receiving a first image stack of a first field of view, the first image stack comprising images captured with different focus from the first field of view;

determining, from the first image stack, a first spatial distribution of focus depths in which different areas in the first field of view are in focus;

determining a first local sample thickness and a first sample tilt in the first field of view based on the first spatial distribution of focus depths,

wherein determining the first local sample thickness and the first sample tilt comprises:

subtracting terms accounting for pre-defined field curvature and/or pre-defined optical axis tilt from a focus depth model;

determining coefficients defining the first local sample thickness and the first sample tilt by applying a linear estimation approach to the focus depth model; and

wherein the focus depth model comprises coordinate functions transforming an image pixel coordinate and a stage control coordinate pair to stage coordinates, and the method further comprises:

capturing, at a location according to the stage control coordinates, an image of a known target located at a location according to the stage coordinates on a calibration slide;

measuring an image pixel location according to image pixel coordinates from the image of the target;

forming a pair of equations modeling a transformation from the image pixel coordinate and the stage control coordinate pair to the stage coordinates;

repeating the capturing, measuring and forming at least five times, wherein the known target is located at different location each time; and

determining coefficients of the transformation by applying a linear estimation approach to the pairs of equations; and the method further comprises

estimating, based on the first local sample thickness and the first sample tilt, a focus setting for capturing a second image stack from a second field of view.

8. An apparatus comprising at least one processor; at least one memory including computer program code; the at least one memory and the computer program code configured to, with the at least one processor, cause the apparatus at least to perform:

receiving a first image stack of a first field of view, the first image stack comprising images captured with different focus from the first field of view;

determining, from the first image stack, a first spatial distribution of focus depths in which different areas in the first field of view are in focus;

determining a first local sample thickness and a first sample tilt in the first field of view based on the first spatial distribution of focus depths;

receiving a third image stack of a third field of view, the third image stack comprising images captured with different focus from the third field of view and wherein the first field of view and the third field of view are adjacent fields of view for a second field of view;

determining, from the third image stack, a third spatial distribution of focus depths in which different areas in the third field of view are in focus;

determining a third local sample thickness and a third sample tilt based on the third spatial distribution of focus depths; and

estimating, based on the first local sample thickness, the first sample tilt, the third local sample thickness and the third sample tilt, a focus setting for capturing a second image stack from the second field of view.

9. The apparatus according to claim 8 , further caused to perform:

estimating a first focus setting based on the first local sample thickness and the first sample tilt;

estimating a third focus setting based on the third local sample thickness and the third sample tilt; and

averaging the first focus setting and the third focus setting to obtain the focus setting for capturing the second image stack from the second field of view.

10. The apparatus according to claim 9 , further caused to perform assigning a weight for the first focus setting based on the distance between the first field of view and the second field of view.

11. The apparatus according to claim 9 , further caused to perform:

assigning a weight for the first focus setting based on a planarity of the first spatial distribution of focus depths.

12. The apparatus according to claim 8 , wherein the apparatus is a digital microscope scanner.

13. The apparatus according to claim 8 , wherein the determining the first local sample thickness and the first sample tilt comprises eliminating effects caused by apparatus-specific measures from the spatial distribution of focus depths.

14. The apparatus according to claim 13 , wherein the apparatus-specific measures comprise one or more of a pre-defined field curvature and a pre-defined optical axis tilt.

15. An apparatus comprising:

at least one processor;

at least one memory including computer program code, the at least one memory and the computer program code configured to, with the at least one processor, cause the apparatus at least to perform:

receiving a first image stack of a first field of view, the first image stack comprising images captured with different focus from the first field of view;

determining, from the first image stack, a first spatial distribution of focus depths in which different areas in the first field of view are in focus;

determining a first local sample thickness and a first sample tilt in the first field of view based on the first spatial distribution of focus depths,

wherein determining the first local sample thickness and the first sample tilt comprises:

subtracting terms accounting for pre-defined field curvature and/or pre-defined optical axis tilt from a focus depth model; and

determining coefficients defining the first local sample thickness and the first sample tilt by applying a linear estimation approach to the focus depth model;

wherein the focus depth model comprises coordinate functions transforming an image pixel coordinate and a stage control coordinate pair to stage coordinates, and the apparatus is further caused to perform:

capturing, at a location according to the stage control coordinates, an image of a known target located at a location according to the stage coordinates on a calibration slide;

measuring an image pixel location according to image pixel coordinates from the image of the target;

forming a pair of equations modeling a transformation from the image pixel coordinate and the stage control coordinate pair to the stage coordinates;

repeating the capturing, measuring and forming at least five times, wherein the known target is located at different location each time; and

determining coefficients of the transformation by applying a linear estimation approach to the pairs of equations; and the apparatus is further caused to perform

estimating, based on the first local sample thickness and the first sample tilt, a focus setting for capturing a second image stack from a second field of view.

16. The apparatus according to claim 15 , wherein the apparatus is a digital microscope scanner.

17. A computer program product embodied on a non-transitory computer readable medium, comprising computer program code configured to, when executed on at least one processor, cause an apparatus or a system to:

receive a first image stack of a first field of view, the first image stack comprising images captured with different focus from the first field of view;

determine, from the first image stack, a first spatial distribution of focus depths in which different areas in the first field of view are in focus;

determine a first local sample thickness and a first sample tilt in the first field of view based on the first spatial distribution of focus depths;

receive a third image stack of a third field of view, the third image stack comprising images captured with different focus from the third field of view and wherein the first field of view and the third field of view are adjacent fields of view for a second field of view;

determine, from the third image stack, a third spatial distribution of focus depths in which different areas in the third field of view are in focus;

determine a third local sample thickness and a third sample tilt based on the third spatial distribution of focus depths; and

estimate, based on the first local sample thickness, the first sample tilt, the third local sample thickness and the third sample tilt, a focus setting for capturing a second image stack from the second field of view.

18. The computer program product according to claim 17 , wherein the determining the first local sample thickness and the first sample tilt comprises

eliminating effects caused by apparatus-specific measures from the spatial distribution of focus depths.

19. The computer program product according to claim 18 , wherein the apparatus-specific measures comprise one or more of a pre-defined field curvature and a pre-defined optical axis tilt.

20. A computer program product embodied on a non-transitory computer readable medium, comprising computer program code configured to, when executed on at least one processor, cause an apparatus or a system to:

receive a first image stack of a first field of view, the first image stack comprising images captured with different focus from the first field of view;

determine, from the first image stack, a first spatial distribution of focus depths in which different areas in the first field of view are in focus;

determine a first local sample thickness and a first sample tilt in the first field of view based on the first spatial distribution of focus depths,

wherein determining the first local sample thickness and the first sample tilt comprises:

subtracting terms accounting for pre-defined field curvature and/or pre-defined optical axis tilt from a focus depth model;

determining coefficients defining the first local sample thickness and the first sample tilt by applying a linear estimation approach to the focus depth model, wherein the focus depth model comprises coordinate functions transforming an image pixel coordinate and a stage control coordinate pair to stage coordinates, and wherein the computer program code is further configured to cause the apparatus or the system to:

capture, at a location according to the stage control coordinates, an image of a known target located at a location according to the stage coordinates on a calibration slide;

measure an image pixel location according to image pixel coordinates from the image of the target;

form a pair of equations modeling a transformation from the image pixel coordinate and the stage control coordinate pair to the stage coordinates;

repeat the capturing, measuring and forming at least five times, wherein the known target is located at different location each time; and

determine coefficients of the transformation by applying a linear estimation approach to the pairs of equations; and the computer program code is further configured to cause the apparatus or the system to

estimate, based on the first local sample thickness and the first sample tilt, a focus setting for capturing a second image stack from a second field of view.